BPDA is a compound useful as a starting material for the production of an aromatic polyimide which is now attracting attention as a heat resistant resin. An aromatic polyimide formed from BPDA can be prepared by either a process for producing it by a polymerization reaction of BPDA and an aromatic diamine or a process for ring-closing imidation of a polyamic acid obtained by polymerization of BPDA and an aromatic diamine at a low temperature in the vicinity of room temperature.
BPDA can be obtained by heating biphenyltetracarboxylic acid (hereinafter referred to as BTC) in a nitrogen atmosphere under a pressure of from atmospheric pressure to a reduced pressure of 40 mmHg at from 100 to 500° C. for a dehydration ring-closing reaction (JP-B-57-15098). However, as a result of a study on a problem that the viscosity cannot sufficiently be increased when it is formed into an aromatic polyimide, it has been found that the cause may be biphenyltricarboxylic acid and/or its anhydride (hereinafter referred to as the “tri-derivative” included in the product. As a means to solve such a problem, a process for obtaining BPDA has been proposed wherein the content of the tri-derivative is reduced to a level of less than 0.2 wt % by carrying out the heating of BTC under specific conditions (U.S. Pat. No. 4,958,002).
Namely, this proposal is characterized in that when BPDA is formed by heating BTC crystals in an atmosphere of an inert gas at a dehydrating temperature of at least 250° C. to form an anhydride, preheating for temperature rise from 80° C. to the dehydrating temperature, is carried out so that the average temperature-rising rate will not be higher than 50° C./hr. In such a manner, water attached to the crystal and crystal water are removed and then the preheated crystals are heated at a dehydration temperature of from 250 to 300° C. for at least three hours for dehydration. Further, in order to realize the heating under the above-mentioned specific conditions, this proposal also proposes to employ a reaction tank (drying machine) which houses a disk-type stirring device having rotary vanes on a horizontal rotary shaft having a hollow portion internally formed to let pressurized steam or a heat resistant heating medium flow therethrough, and which has an outer wall substantially covered by a jacket to let pressurized steam or a heat resistant heating medium flow therethrough.
However, as is evident from Examples in U.S. Pat. No. 4,958,002, it takes a long time exceeding 9 hours even only for the time required for the preheating, although BPDA of high purity with a tri-derivative content of from 0.06 to 0.07 wt % is obtained. If the time required for the dehydration ring-closing reaction is further added, the process requires a long time, and a solution is desired to increase the productivity of the process while the high purity is maintained.